1. Field of the Invention
The present invention relates generally to a circuit board such as an IC board, and more particularly, to a circuit board constructed using a functionally gradient material.
2. Description of the Prior Art
Conventionally, circuit boards made of various materials have been used so as to construct an electronic circuit. Of the circuit boards, a high power IC board and a power module board are constructed using a material superior in thermal conductivity because a circuit element carried thereon generates a great amount of heat.
One example of the circuit boards constructed using, for example, a metal material is shown in FIG. 6. A metal composite board 1 shown in FIG. 6 has a structure in which a metal plate 1a made of aluminum or the like, a synthetic resin layer 1b made of epoxy resin or the like, and a copper foil 1c are laminated. In the metal composite board 1, heat generated in a circuit element carried thereon is effectively radiated utilizing high thermal conductivity of the metal plate 1a. In addition, the metal composite board 1 is superior in mechanical strength to a ceramic board because it uses the metal plate 1a as a base material.
However, the metal composite board 1 has the disadvantage in that it is not sufficiently high in heat resistance so that it is liable to be softened at a temperature of a maximum of 300.degree. C. because the synthetic resin layer 1b is provided so as to achieve electrical insulation.
Moreover, since the metal composite board 1 has the above described laminated structure, it also has the disadvantage in that thermal stress is concentrated on the boundary between the layers due to the difference in coefficients of thermal expansion of materials of the respective layers, so that stripping or the like is liable to occur in the boundary between the layers due to heat shock.